Re: Low loss receive filter for 23cm

ian hope (2E0IJH)

John over at Colchester Aerial parts does a 1296 intermod filter,

Sent: Monday, September 26, 2016 at 7:33 AM
From: "DougF uwaves@... [ukmicrowaves]"
To: ukmicrowaves@...
Subject: Re: [ukmicrowaves] Low loss receive filter for 23cm


Hello Neil and other readers.

If you're willing to engage in a bit of precision metal work (which I do like doing!), I can recommend the solid copper-and-brass interdigital filter that was first described in the 'QST' magazine for March 1968 - yes, nearly 50 years ago!  That article focuses more on such filters for 432 MHz, but does include dimensions for one for 1296 MHz.

I have one here and, in acknowledgement of your 'problems' possibly being as far away as 600 MHz, I have today measured its performance down that far - insertion loss across a broad range of frequency - and the results of my testing were:

MHz     I.L. (dB)
1309     0.2  (the minimum)
1296     0.3
1290     0.4
1280     0.5
1260     1.1
1240     4.1
1220     9.0
1200     13.6
1180     19.5
1160     22.9
1140     25.1
1120     29.5
1100     33.1
1000     42.2
  900     50.5
  800     56.6
  700     57.2
  600     61.5
The pattern further above 1300 MHz is very much a mirror image of the above measurements.  The return loss looking into each port (with the other port terminated) is very good.

I trust that these data may be of assistance - possibly to initiate another project!

If you have difficulty accessing the original article (which must have been copied many times over the years), do get back to me and I should be able to scan and post somewhere Ed's hand-drawn design notes (which are essentially the same as the article), thus avoiding violation of QST's copyright.

And I could post an image or two of the completed filter.

Best 73,

--Doug Friend, VK4OE.

p.s.  I was fortunate a decade or more ago to have been given the precisely-cut elements for this filter by the (now) late Eddie Penikis VK1VP.  Ed had an inspiring ability to work precisely in everything that he set his hands to do - all I had to do was to build the housing, which I did in solid brass.  As you'll see, it works wonderfully well!

p.p.s.  I immediately also thought of the nice off-the-shelf filter made by a German fellow and recommended by the Kuhne company for use with their 28 MHz to 1296 MHz transverters.  However, not having one here to test, I can't vouch for it having similar broadband performance to the 'old' design over the same frequency range shown above.

On 26/09/2016 08:25, Neil neil@... [ukmicrowaves] wrote:

I have major problems with overload of my 23cm VLNA when my mast is
fully extended. Not sure if it is digi TV or something else, but I get
around a dozen sharp peaks of noise from many directions, some more than
30dB over band noise. I'm using a 44-ele Wimo. If I insert 3/6/10dB
attenuators before the VLNA, there is very little improvement. With 20dB
attenuation, the problem goes away almost completely, but so do most DX

Looking at the output of the VLNA on a spectrum analyser, I see 15-20MHz
wide rounded noise peaks between 1.15 and 1.35 GHz in some directions
with the mast up high. I can also see some *very* big signals (-20dBm
according to the analyser) in the 1.2 GHz range which shift about a lot,
too fast to see on the analyser other than occasional spikes.

With the mast lower, the problem is much reduced. I built a W6PQL-style
two-post bandpass filter in a diecast ally box, using 10mm copper tube
posts and brass tuning screws, with coupling loops on SMAs, but I am
unable to get the insertion loss less than 0.65dB. Also the loss is
very sensitive to the contact between the case and lid. Using a couple
of G-cramps to jam the lid shut helps a bit. I suppose machining an
enclosure from aluminium bar and using a lot of screws on the lid would
help, but it all seems a bit sensitive and delicate and lots of screw
thread to metal connections to go off-tune. Would a PCB stripline filter
or some sort of machined coaxial filter do a similar job? I can
probably manage with less stop-band rejection so long as the insertion
loss is tiny.

With the filter in circuit before the VLNA, the QRM problem is cured
*completely*, so the problem is definitely out of band overload.

Question: What is the lowest loss filter I can make/buy which will drop
600-800MHz TV signals and cellphone stuff by at least 20dB, but have
minimal effect on 23cm signals and stay nice and stable? No point in
having a super VLNA if I'm losing 0.6dB or more in a filter. Happy to
use HPF, band-reject, bandpass, whatever will give me enough rejection
but lowest possible insertion loss. As the problem only happens for
terrestrial use, is there any point in trying to achieve better than say
1.2dB system NF at 23cm? (VLNA 0.45dB-ish, 1.5m total coax, big Radiall
coax relay, 3 x N plug/sockets, 3 x SMA plug/sockets and filter ahead of
the VLNA).

I've read the splendid ITU noise doc several times...!!PDF-E.pdf

Neil G4DBN


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